Do you have actual examples of how that would be useful to you?
On Monday, September 10, 2018 at 9:45:12 PM UTC-4, Larry Clapp wrote:
>
> The conversation here is deep, and I haven't followed all of it. I
> *think* y'all are discussing some different contract system than the
> actual draft proposal?
>
> If that's the case:
>
> You can't express field-accessors, which is an actual, inarguable
>> reduction in power that I'm perfectly fine with
>
>
> How would you instantiate a generic function that wanted to access field F
> of a generic type T?
>
> if you can't do that, well, I'm not fine with that.
>
> But I may have misunderstood.
>
> — L
>
> On Monday, September 10, 2018 at 7:45:04 PM UTC-4, Axel Wagner wrote:
>>
>> The unambiguous cases aren't ambiguous of course. It's the ambiguous
>> cases I'm concerned about :) My post and this thread contain a bunch of
>> those. They are mostly growing out of builtin functions and things like
>> `range` statements and index-expressions.
>>
>> You can translate all of my "pseudo-interfaces" into a clear and short
>> contract
>>
>> contract comparable (v T) { v == v } // also allows !=
>> contract ordered (v T) { v < v } // also allows <=, >, >=
>> contract boolean (v T) { v || v } // also allows &&, !
>> contract bitwise (v T) { v & v } // also allows |, ^, <<, >>
>> contract arith (v T) { v * v } // also allows +, -, /
>> // well, this one's a bit harder…
>> contract concat (v T) { v + v; v == "" } // also allows range,
>> index[int], copy
>> contract complex (v T) { real(v) } // also allows imag(v)
>> contract nilable (v T) { v == nil } // also allows != nil
>>
>> There is some discussion about maps, slices and channels to be had - but
>> I think the vast majority of interesting cases can be covered by taking
>> e.g. a `chan T`. A defined type with underlying type `chan T` is assignable
>> to that, so it would appear a perfectly fine way to express this contract
>> contract Chan (ch C, el T) { var x T = <-ch; ch <- x }
>> (and correspondingly for send/recv-only channels).
>>
>> You can't express field-accessors, which is an actual, inarguable
>> reduction in power that I'm perfectly fine with (similarly to how we're
>> fine with interfaces not being able to express that).
>>
>> Which IMO brings up the question: If we can express the vast majority (or
>> even all) of the needed contracts as combinations of this handful of
>> base-cases, why would we need to allow the full power of Go syntax, which
>> enables to write all the less than obvious ones too?
>>
>> (to re-emphasize: All of this isn't polished. I did sanity-check against
>> the examples in the contract design doc, but I have not put enough thought
>> into it that there might not be a couple nooks and crannies I haven't
>> thought of, as you've proven before :) )
>>
>> On Tue, Sep 11, 2018 at 1:21 AM Jonathan Amsterdam <[email protected]>
>> wrote:
>>
>>>
>>>
>>> On Monday, September 10, 2018 at 4:17:57 PM UTC-4, Axel Wagner wrote:
>>>>
>>>> On Mon, Sep 10, 2018 at 8:57 PM Jonathan Amsterdam <[email protected]>
>>>> wrote:
>>>>
>>>>> FWIW, I think Ian's criticism of not wanting a list of new identifiers
>>>>>> to express operator constraints is fair. It is a genuine roadblock to c)
>>>>>> and if we're dead set of setting that as a baseline requirement, I agree
>>>>>> that a declarative/interface-based approach won't work.
>>>>>>
>>>>>
>>>>> I don't understand. Why are names so important? Why couldn't you use
>>>>> "T == T" to mean "T is comparable"? Or "To(From)" to mean "From is
>>>>> convertible to To"?
>>>>>
>>>>
>>>> It's not the name that is important, it's the declarative nature. I and
>>>> other people have already gone into detail with the problems we are having
>>>> with using imperative constructs to define constraints (mainly that they
>>>> are ambiguous and that it's hard both to enumerate the sets allowed by a
>>>> contract and to define a suitable contract for an intended set of
>>>> constraints).
>>>>
>>>
>>> I completely agree with you there (although I find the
>>> declarative/imperative terminology confusing). I think that except for
>>> interface-like constraints, we should be constraining by broad properties
>>> of types like comparable, ordered and numeric, rather than specific
>>> operations.
>>>
>>>>
>>>>
>>> I don't care if the declarative instruction is called "flooglehorn" or
>>>> "comparable". But I do care that it's an an actual declaration, mapping
>>>> 1:1
>>>> in a clear way to intent. Not an ambiguous statement from Go's current
>>>> Grammar.
>>>>
>>>
>>> It wouldn't be ambiguous. The spec would say "in type constraints, `T ==
>>> T` means that T is comparable". I think people would learn to understand
>>> that, just as they understand that for a type T, `*T` means "pointer to T,"
>>> not "dereference T".
>>>
>>>
>>>
>>>
>>>>
>>>>
>>>
>>>> Personally, I don't think that should be a requirement. Personally I
>>>>>> think it's worth adding extra identifiers, if we get declarative
>>>>>> constraint-specs for that - but that's just my opinion and everyone has
>>>>>> one
>>>>>> of those.
>>>>>>
>>>>>> But the issues you are bringing up are IMO not "fundamental'. a) to
>>>>>> c) from above aren't really touched by your criticism, AIUI. To me, they
>>>>>> seem like issues of syntax and how to phrase the spec.
>>>>>>
>>>>>> But if generics are to have operator constraints like T == T, then
>>>>>>> something in the language has to change. Either not all interfaces are
>>>>>>> types, or some interfaces have methods that can't be called, or Go
>>>>>>> operators can have operands of different types. These changes are not
>>>>>>> minor
>>>>>>> tweaks: they alter fundamental aspects of the language.
>>>>>>>
>>>>>>> Contracts, on the other hand, are purely additive. They only come
>>>>>>> into play when writing generic code. If I'm not mistaken, the draft
>>>>>>> design
>>>>>>> doesn't change or even add anything to non-generic Go. There is
>>>>>>> something
>>>>>>> attractive about that orthogonality.
>>>>>>>
>>>>>>
>>>>>> I agree. I think that's fair. I don't think for that they need to be
>>>>>> imperative specifications though.
>>>>>>
>>>>>> (or, allow embedding interfaces into contract-declarations, remove
>>>>>>>> the "type-checking function body" idea and instead define a set of
>>>>>>>> base-contracts you can use for operators) and you'd end up with pretty
>>>>>>>> much
>>>>>>>> my design.
>>>>>>>>
>>>>>>>
>>>>>>> That doesn't sound like your original design at all.
>>>>>>>
>>>>>>
>>>>>> You need to squint harder :) From the rest of your mail, ISTM that we
>>>>>> are putting emphasis on different aspects of my description and the
>>>>>> contracts design. What I was trying to say is that IMO the things about
>>>>>> my
>>>>>> design I like and the things about the contract design you like can
>>>>>> probably be reconciled.
>>>>>>
>>>>>> The first seems problematic, because for multi-parameter contracts
>>>>>>> you wouldn't know which type the parameter referred to.
>>>>>>>
>>>>>>
>>>>>> FWIW (we are now getting lost in ifs-and-buts and it's no longer
>>>>>> clear what the specific ideas are we are talking about), in my original
>>>>>> design as well as that ad-how handwaving you're quoting, constraints
>>>>>> always
>>>>>> apply to a single type and you'd use parametric interfaces (or…
>>>>>> interfaces+) to express simultaneous restrictions.
>>>>>>
>>>>>> But FTR, I did not intend to start an actual discussion around that,
>>>>>> its far too underspecified for that. I was sincere when I said your
>>>>>> criticism is fair and that I had to think about it. My handwaving was
>>>>>> just
>>>>>> to explain why I don't think it can justifiable be called a
>>>>>> *fundamental* issue.
>>>>>>
>>>>>>
>>>>>>> The second seems reasonable to me. Now we can talk about issues like
>>>>>>> whether this adds too many names to the language, or whether you've
>>>>>>> described all the important constraints (I think conversion and
>>>>>>> assignability are important, for instance).
>>>>>>>
>>>>>>
>>>>>> Again, the caveat of handwaving still applies (IMO we should
>>>>>> constrain ourselves to talk about sufficiently spelled out designs -
>>>>>> Ian's
>>>>>> contract design qualifies and I'd also feel fine with the thing I wrote
>>>>>> down in my blog, as long as we agree that its conditional on polishing
>>>>>> the
>>>>>> "are pseudo-interfaces usable as types" question), but: Given that we
>>>>>> have
>>>>>> type-parameters, adding that is fairly straightforward in the form of
>>>>>> (in
>>>>>> the words of my blog post) parametric pseudo-interfaces
>>>>>> "convertible{To,From}(T)" and "assignable{To,From}(T)".
>>>>>>
>>>>>> But yeah, talking about in too much depth here would IMO constitute
>>>>>> high-jacking of threads about Ian's design. I'm also totally cool to
>>>>>> start
>>>>>> a new thread about this after I had time to incorporate your feedback.
>>>>>> Unfortunately this isn't my job, so I have to find time between other
>>>>>> things :)
>>>>>>
>>>>>> Anyway. I think I've been ranty enough for today :)
>>>>>>
>>>>>> On Sunday, September 9, 2018 at 5:21:28 PM UTC-4, Axel Wagner wrote:
>>>>>>>>
>>>>>>>> I don't think saying that is is productive. contracts are more than
>>>>>>>> just "identifiers used as constraints", they are also a syntactic
>>>>>>>> construct
>>>>>>>> to specify those. I specifically don't allow that and that's the whole
>>>>>>>> point I'm making. So this doesn't seem like a particularly nice way to
>>>>>>>> have
>>>>>>>> a discussion.
>>>>>>>>
>>>>>>>> But yes, if it makes you happier, we can call them "contracts",
>>>>>>>> allow to embed them into interfaces and remove contract declarations
>>>>>>>> from
>>>>>>>> the design (or, allow embedding interfaces into contract-declarations,
>>>>>>>> remove the "type-checking function body" idea and instead define a set
>>>>>>>> of
>>>>>>>> base-contracts you can use for operators) and you'd end up with pretty
>>>>>>>> much
>>>>>>>> my design.
>>>>>>>>
>>>>>>>> On Sun, Sep 9, 2018 at 11:17 PM Jonathan Amsterdam <
>>>>>>>> [email protected]> wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Sunday, September 9, 2018 at 3:19:16 PM UTC-4, Axel Wagner
>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>> On Sun, Sep 9, 2018 at 8:49 PM Jonathan Amsterdam <
>>>>>>>>>> [email protected]> wrote:
>>>>>>>>>>
>>>>>>>>>>> The problem is that this program seems to type-check, but it is
>>>>>>>>>>> invalid. The == operator is specified to work on operands of the
>>>>>>>>>>> same type,
>>>>>>>>>>> and it is being used on operands of different types.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Good point. I have to think about it. An ad-hoc solution, FWIW,
>>>>>>>>>> would be to only take into account (or allow) pseudo-interfaces for
>>>>>>>>>> type-constraints.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> The draft design has a name for those: contracts.
>>>>>>>>>
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>>>>>
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